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Abstract

Mesophotic Coral Ecosystems (MCE) represent a new realm of unexplored habitats that range from 30-100m deep. Given the worldwide coral reef decline, MCE research has created great expectations because of their potential as refugia and as a viable source of larvae and nursery for commercial and endangered reef species. Within these deep reef systems sponges play an important ecological role in terms of abundance and richness of species, coupling water column productivity to the benthos and providing rugosity, complexity and refuge for other species. However, information regarding the taxonomic composition and ecology of sponges in MCE is scarce. The main goal of this study was to characterize MCE distribution in five areas of Puerto Rico (La Parguera, Guánica, Vieques, Desecheo and Bajo de Sico) with special emphasis on coral and sponge species. Thirteen (km-long) photo-transects obtained with the Seabed Autonomous Underwater Vehicle (AUV) provided the 1116 images that were analyzed, covering over 3.7 km2, to derive percent cover and species richness for eight categories (sponges, corals, gorgonians, black corals, algae, other, unknown, and abiotic cover). The possible influence of several factors (location, transect, depth, geomorphology, water turbidity, distance from land, chlorophyll a concentration, level of analysis) in the distribution of MCE communities was statistically evaluated with ANOSIM, SIMPER, NMDS, cluster and regression analyses. Sponge data were further analyzed in terms of species richness, color and morphology. Slope is a determinant factor for community composition benthic group dominance and sponge species composition, but not for coral species composition. Coral cover, macroalgal cover and total live cover tend to increase with distance from land and decrease with water turbidity. Depth is an indirect factor affecting MCE ecology. Regression analysis results suggest that effects of the studied factors on MCEs are stronger with depth and that MCEs between 50-100m depth are possibly more sensitive to these factors. Results suggest that in the upper mesophotic range, coral reef characteristics represent an extension of the shallow coral reefs. The lower mesophotic range is subject to low light regimes and perhaps higher sedimentation rates. These factors are limiting to coral growth resulting in sparse plate-like colonies and favoring development of other groups such as black corals, sponges or algae. For this reason it may be more useful to refer to these ecosystems as MREs (Mesophotic Reef Ecosystems), where corals are not dominant. The gorgonian to black coral transition, together with the Agaricia-dominance transition pattern and a coral-to-sponge dominance transition may be indicating a change from euphotic to mesophotic ecosystems. The number of sponge species found (77) is higher than that of corals (28), but in both cases species richness tend to decrease with increasing depth. Morphology of sponges seems to be related to depth and location, whereas color seems to be related to geomorphology, turbidity and location. Sponges with carotenoid pigmentation are the most abundant and widely distributed. Tube, cup-like and massive forms not only provide rugosity and microhabitats for other organisms, but they may conform the most adaptive morphologies to an optimal water-circulation in MCEs. Branching and erect forms may represent an adaptation to steep slopes, while encrusting morphologies are probably opportunistic.